Java Constants

Types of Constants

final int MAX_SCORE = 100;
final String GRADE_A = "Excellent";

public static final double TAX_RATE = 0.08;
public static final String COMPANY = "TechCorp";

final double PI = 3.14159;
final int DAYS_IN_WEEK = 7;

final String APP_NAME = "MyApp";
final String VERSION = "1.0.0";

🔹 Creating Constants

Use the 'final' keyword to create constants that cannot be changed:

public class CreateConstants {
    // Class-level constants (accessible to all methods)
    public static final String APP_NAME = "Student Management System";
    public static final double VERSION = 2.1;
    public static final int MAX_USERS = 1000;
    
    public static void main(String[] args) {
        // Local constants (only accessible in this method)
        final int PASSING_GRADE = 60;
        final String SEMESTER = "Fall 2024";
        final boolean DEBUG_MODE = true;
        
        // Using constants
        System.out.println("Application: " + APP_NAME);
        System.out.println("Version: " + VERSION);
        System.out.println("Max Users: " + MAX_USERS);
        System.out.println("Semester: " + SEMESTER);
        System.out.println("Passing Grade: " + PASSING_GRADE);
        System.out.println("Debug Mode: " + DEBUG_MODE);
        
        // Calculate something using constants
        int studentScore = 75;
        if (studentScore >= PASSING_GRADE) {
            System.out.println("Student passed with score: " + studentScore);
        } else {
            System.out.println("Student failed with score: " + studentScore);
        }
    }
}

🔹 Why Use Constants?

Constants provide several benefits in your programs:

public class ConstantBenefits {
    // Instead of using "magic numbers" throughout code
    public static final double SALES_TAX = 0.0825;  // 8.25%
    public static final int DISCOUNT_THRESHOLD = 100;
    public static final double BULK_DISCOUNT = 0.10; // 10%
    
    public static void main(String[] args) {
        double itemPrice = 150.0;
        int quantity = 3;
        
        double subtotal = itemPrice * quantity;
        System.out.println("Subtotal: $" + subtotal);
        
        // Apply bulk discount if applicable
        if (subtotal >= DISCOUNT_THRESHOLD) {
            double discount = subtotal * BULK_DISCOUNT;
            subtotal = subtotal - discount;
            System.out.println("Bulk discount applied: -$" + discount);
            System.out.println("After discount: $" + subtotal);
        }
        
        // Calculate tax
        double tax = subtotal * SALES_TAX;
        double total = subtotal + tax;
        
        System.out.println("Tax (" + (SALES_TAX * 100) + "%): $" + tax);
        System.out.println("Final total: $" + total);
    }
}

🔹 Constant Naming Convention

Follow these naming rules for constants:

public class ConstantNaming {
    // ✅ Good constant names (UPPER_CASE with underscores)
    public static final int MAX_ATTEMPTS = 3;
    public static final String DEFAULT_COLOR = "blue";
    public static final double CONVERSION_RATE = 2.54;
    public static final boolean ENABLE_LOGGING = true;
    
    // ❌ Poor constant names (avoid these styles)
    // public static final int maxAttempts = 3;     // Should be UPPER_CASE
    // public static final String defaultcolor = "blue"; // Missing underscore
    // public static final double rate = 2.54;     // Not descriptive enough
    
    public static void main(String[] args) {
        // Using well-named constants makes code self-documenting
        for (int attempt = 1; attempt <= MAX_ATTEMPTS; attempt++) {
            System.out.println("Attempt " + attempt + " of " + MAX_ATTEMPTS);
            
            if (attempt == 2) {
                System.out.println("Success on attempt " + attempt + "!");
                break;
            }
        }
        
        System.out.println("Default color: " + DEFAULT_COLOR);
        System.out.println("Inches to CM rate: " + CONVERSION_RATE);
        System.out.println("Logging enabled: " + ENABLE_LOGGING);
        
        // Convert inches to centimeters using constant
        double inches = 5.0;
        double centimeters = inches * CONVERSION_RATE;
        System.out.println(inches + " inches = " + centimeters + " cm");
    }
}

🔹 Constants vs Variables

Understanding the difference between constants and variables:

public class ConstantsVsVariables {
    // Constant - value never changes
    public static final String COMPANY_NAME = "TechSolutions Inc.";
    
    public static void main(String[] args) {
        // Variable - value can change
        int employeeCount = 50;
        String currentProject = "Website Redesign";
        
        // Constant - value cannot change
        final double HOURLY_RATE = 25.50;
        
        System.out.println("Company: " + COMPANY_NAME);
        System.out.println("Employees: " + employeeCount);
        System.out.println("Current project: " + currentProject);
        System.out.println("Hourly rate: $" + HOURLY_RATE);
        
        // We can change variables
        employeeCount = 55;  // ✅ This works
        currentProject = "Mobile App Development";  // ✅ This works
        
        System.out.println("\nAfter updates:");
        System.out.println("Employees: " + employeeCount);
        System.out.println("Current project: " + currentProject);
        
        // We CANNOT change constants
        // HOURLY_RATE = 30.0;  // ❌ This would cause a compilation error!
        // COMPANY_NAME = "New Company";  // ❌ This would also cause an error!
        
        // Calculate weekly pay using constant
        int hoursWorked = 40;
        double weeklyPay = hoursWorked * HOURLY_RATE;
        System.out.println("Weekly pay for " + hoursWorked + " hours: $" + weeklyPay);
    }
}